1. Field of the Invention
The invention relates to a defected ground structure. Particularly, the invention relates to a defected ground structure with a shielding effect.
2. Description of Related Art
With development of wireless radio frequency (RF) and microwave communication systems, 3G and 3.5 G communication systems are widely used in the industry, and a 4 G communication system is under development. A wireless communication system generally includes a transmitting module and a receiving module comprising a power amplifier, a mixer, a voltage control oscillator and a bandpass filter, etc. The filter is located at a next stage of an antenna. Signal received by the antenna is transmitted to the filter, and the filter keeps the signals of required band and isolates the signals of unnecessary bands. Therefore, a frequency response quality of the filter directly influences correctness of signal processing of post-end circuits, and a performance of the filter determines the quality of the wireless communication system.
Since a planar micro strip filter has advantages of small size, easy fabrication and low cost, it is widely used in current wireless communication systems. In a stepped impedance resonator (SIR) structure provided by Makimoto in 1980, a microstrip resonator is designed in different widths to have different characteristic impedances, so that by adjusting a ratio of the widths (the characteristic impedances) of two sections of the resonator, an excellent characteristic of controlling double and triple base-frequency harmonic positions can be achieved.
A defected ground structure (DGS) is disclosed by South Korean scholars Park et al. in the late 1990s. The so-called “defected” refers to specific pattern units etched on a metal ground surface of a printed circuit board, which are generally a set of symmetric units or a plurality of periodically arranged units. The DGS disturbs a current distribution and changes an equivalent distributed inductance and capacitance of the original circuit. When an electromagnetic wave propagates on the DGS with periodic obstacles, energy (or frequency) thereof only appears at some specific wave vectors, so that the energy is suppressed at some frequency bands to achieve a special bandstop effect. Such characteristic of the DGS is widely used for designing microwave devices such as low-pass filters, and bandpass filters, etc.
Since in the wireless communication system, passive devices may occupy 65% of a total area of a RF front-end circuit, and the passive devices such as the filter, etc. are not easy to be integrated into a silicon substrate, it is still a development and research focus for designing a filter with small size, low cost, high selectivity and high performance.